Various sources of magnetic fields are encountered commonly in everyday life. These sources generally operate at extremely low frequencies, typically 50-60 Hz, and include electric power transmission and distribution lines, transformers and various appliances. While it may take several years before scientific studies conclusively prove the absence or the presence of health effects of such low frequency magnetic fields, public concern nevertheless currently exists.
There are many configurations for transmitting electric power, four of which are shown in FIGS. 1 and 2. These four configurations generally give rise to magnetic field values which are smaller than other conventional configurations. Typically, the known power transmission systems employ a delta arrangement of three conductors to conduct power in a three phase manner, such as shown in FIGS. 1a and 1b, or a superbundle and a low reactance double circuit, such as shown in FIGS. 2a and 2b. The magnetic field values which are generated by such configurations are tabulated below for comparison. The lowest conductor is assumed to be positioned at 14 meters (m) from the ground and the distance of separation between the conductors pertaining to different phases is kept at 10 meters (m). The edge of the right of way is at 30 meters (m) from the centerline of the configuration. These are the distances recommended for 500 kV electric power lines ["Transmission Line Reference Book, 345 kV and Above," J. J. LaForest, ed., 2nd Edition, Electric Power Research Institute (EPRI), Palo Alto, Calif.: 1987]. The time dependence of the currents flowing in phases A, B, and C at a frequency of 60 Hz are: EQU I.sub.A =I sin (2.pi.60t+2.pi./3) (1) EQU I.sub.B =I sin (2.pi.60t) (2) EQU I.sub.C =I sin (2.pi.60t-2.pi./3) (3)
where I.sub.A is the current passing through the conductor with phase A at time t. Similarly I.sub.B and I.sub.C are the currents for phases B and C respectively. I is the magnitude of the current and will be taken here as 1000 A. The magnetic field values given in the table are calculated numerically by using the EPRI computer program ENVIRO. Here, the resultant magnetic field vector is elliptically polarized. B.sub.max represents the maximum magnetic field corresponding to the major axis of the ellipse.
TABLE 1 ______________________________________ Comparison of magnetic fields generated by representative single and double circuit configurations B.sub.max at the edge of the Configuration FIG. # right of way (mG) ______________________________________ Horizontal delta, single circuit 1a 17.1 Inverted delta, single circuit 1b 15.5 Super bundle, double circuit 2a 47.2 Low reactance, double circuit 2b 12.0 ______________________________________
Therefore, the magnetic field magnitude values generated by various known electric power transmission configurations are high, i.e., generally in the range of 12.0 to 47.2 mG at the edge of the right of way. Thus, a need exists for electric power transmission and distribution lines, which generate considerably smaller magnetic field intensity values at the edge of the right of way and beyond.
In the case of current bearing magnetic field sources like the power transmission lines, the source is well characterized. Thus, the source characteristics, including the magnitude and phase angle of the currents of various conductors, can be used to completely describe the magnetic field distributions at all locations surrounding the source. On the other hand, there are many other examples of sources of low frequency magnetic fields used in power transmission and distribution with unknown or difficult to characterize current distributions. A good example is a transformer unit. For such cases, an additional need exists to develop a method for mitigating their magnetic fields.
Accordingly, it is an object of the invention to reduce the magnetic field magnitude at certain preferred locations along a right of way of an electrical power system.
It is another object of the invention to mitigate the magnetic field at a predetermined point spaced from a source producing the magnetic field.
It is another object of the invention to create a power transmission system so as to mitigate a magnetic field at predetermined points along a right of way.